1. Technical Field
This invention relates generally to memory devices, and more particularly, to resistive memory device operation.
2. Background Art
FIG. 1 illustrates a type of resistive memory device 30 known as an ion motion resistive memory device. The memory device 30 includes an electrode 32 (for example copper), a copper sulfide layer 34 on the electrode 32, an active layer 36, for example a copper oxide layer, on the layer 34, and an electrode 38 (for example titanium) on the active layer 36. Initially, assuming that the memory device 30 is unprogrammed, in order to program the memory device 30, ground is applied to the electrode 38, while a positive voltage is applied to electrode 32, so that an electrical potential Vpg (the “programming” electrical potential) is applied across the memory device 30 from a higher to a lower electrical potential in the forward direction of the memory device 30 (see FIG. 2, a plot of memory device current vs. electrical potential applied across the memory device 30). This potential is sufficient to cause copper ions to be attracted from the layer 34 toward the electrode 38 and into the active layer 36 (A) so that conductive filaments are formed, causing the active layer 36 (and the overall memory device 30) to be in a (forward) low-resistance or conductive state. Upon removal of such potential (B), the ions drawn into the active layer 36 during the programming step remain therein, so that the active layer 36 (and memory device 30) remain in a conductive or low-resistance state.
In the read step of the memory device 30 in its programmed (conductive) state, an electrical potential Vr (the “read” electrical potential) is applied across the memory device 30 from a higher to a lower electrical potential in the forward direction of the memory device 30. This electrical potential is less than the electrical potential Vpg applied across the memory device for programming (see above). In this situation, the memory device 30 will readily conduct current, which indicates that the memory device 30 is in its programmed state.
In order to erase the memory device, a positive voltage is applied to the electrode 38, while the electrode 32 is held at ground, so that an electrical potential Ver (the “erase” electrical potential) is applied across the memory device 30 from a higher to a lower electrical potential in the reverse direction of the memory device 30. This potential is sufficient to cause copper ions to be repelled from the active layer 36 toward the electrode 32 and into the layer 34 (C), causing the active layer 36 (and the overall memory device 30) to be in a high-resistance or substantially non-conductive state. This state remains upon removal of such potential from the memory device 30.
In the read step of the memory device 30 in its erased (substantially non-conductive) state, the electrical potential Vr is again applied across the memory device 30 from a higher to a lower electrical potential in the forward direction of the memory device 30, as described above. With the active layer 34 (and memory device 30) in a high-resistance or substantially non-conductive state, the memory device 30 will not conduct significant current, which indicates that the memory device 30 is in its erased state. A resistive memory array typically includes a large number of these memory devices 30, each of which can be individually read, programmed and erased.
As will be noted from the above description and FIG. 2, relatively low current is required for reading the state of the memory device 30 (desirable to apply low voltage so as to avoid disturb into another state), while substantially higher currents are required for programming and erasing the device 30. In turn, architecture for proving such high currents is required.
Additionally, typically, in a conventional flash memory array, a two-command approach is undertaken, wherein, upon a first command, a whole page of data is erased, and then upon a second command data is written into the array. It would be desirable to provide an approach wherein, upon a single command, erasing and programming is undertaken on only those the memory devices wherein a change of state is required from the previous state, i.e., those memory devices whose state is not to be changed from the previous state are left in that state and do not undergo an erase or program operation.
Therefore, what is needed is a method of meeting the high current needs in programming and erasing the memory devices of resistive memory devices, meanwhile using a single command and changing the state of only those memory devices wherein a change of state is required.